Although only 6% of the eligible United Kingdom population regularly donate blood, reasons for low donation rates remain poorly understood. In a grounded theory investigation, blood donors (n = 23) and non-donors (n = 27) completed a questionnaire that included 15 open-ended items. Semi-structured interviews were conducted with sub-samples of donors (n = 5) and non-donors (n = 7). Analysis of open-ended responses and interview transcripts identified five themes. Three themes, namely pro-social views, helping behaviour and awareness had similar relevance for donors and non-donors. Two themes, anxiety and practical difficulties, were used by non-donors to account for non-donation despite intentions to donate. Although non-donors' anxieties may be difficult to overcome, perceived practical difficulties provide scope for intervention. The removal of perceived barriers to donation offers one way of increasing donation rates.